AU-SN/W AND AU-SN/CR METALLIZED CHEMICAL-VAPOR-DEPOSITED DIAMOND HEAT SINKS FOR INP LASER DEVICE APPLICATIONS

An attempt to apply a more stable metallurgical soldering system than the traditionally used Ti/Pt/Au-Sn reactive system was explored by replacing the barrier metals in between the chemical vapor deposited diamond (CVDD) submount and the Au-Sn solder with either W or Cr layers. In the case of the W layer a thin layer of Ti was added to improve the adhesion of Au-Sn solder to the W layer. The CVDD/W(100 nm)/Ti(10 nm)/Au-Sn(1.5-2.2 mu m) system was heat treated at 350 degrees C for durations of 5 s-5 min, simulating a bonding cycle, and the metallurgical interactions were analyzed. Some diffusion of the Ti was observed, but the W layer remained intact. Thus, due to the absence of interaction between the Sn or Au and the W barrier layer, the Au:Sn ratio was kept uniform for long duration heating (> 2 min). The Au-Sn solder tends to ''ball up'' in local spots leading to the formation of local voids in the continuous solder layer over the W. In the case of the CVDD/Ti (100 nm)/Cr(200 nm)/AuSn(1.5-2.2 mu m) structure a minimum interaction between the Au-Sn solder and the barrier was observed through a heating cycle at 350 degrees C for 5 min. This heat treatment, however, led to solder surface roughness in the order of about 5000 Angstrom between bump peak and valley, but the Au:Sn composition remained in the eutectic ratio of 70:30 at. %. Both soldering metallurgical systems gave a potential good bonding of InP laser diodes to CVDD heat sinks.